Apparatus and method for recording an information on a recordable optical record carrier using oval spot profile

ABSTRACT

To obtain a higher recording density, and thus higher data capacity, it is proposed according to the present invention to reduce the numerical aperture of the optical means ( 3, 4 ) in the direction orthogonal to the information recording direction, which is the radial direction for an optical disc, during recording of information to obtain a light beam having a substantially oval spot profile having a shorter axis in the information recording direction, i.e. the tangential direction for an optical disc, compared to the direction orthogonal that to, i.e. the radial direction. Recording marks having a corresponding oval profile arec achieved.

The present invention relates to a an apparatus for recording aninformation on a recordable optical record carrier by irradiation of alight beam onto said record carrier for forming marks and landsrepresenting said information along an information recording direction.The present invention relates further to a corresponding recordingmethod, an optical record carrier and a computer program forimplementing said method.

The total data capacity of optical discs is determined by the radial andtangential data density. The radial density is determined by the datatrack pitch, the tangential data capacity by the shortest mark that canbe written. For rewritable phase-change discs, re-crystallization at thetrailing edge of a mark during writing of the next mark is used toobtain a mark of shorter length than the optical spot size. Typically,half of the mark is erased to end up with a crescent shaped mark. Forthe recently introduced Blu-ray Disc (BD), a total data capacity of 25Gbyte can be recorded on a single recording layer of a 12 cm disc. Theshortest length is 150 nm (d=1 code) while the optical spot is 300 nm inwidth (1/e radius of the optical spot is 150 nm).

A recordable (write-once) BD format is required in addition to the RW(rewritable) format. This so-called BD-R system should also achieve thehigh data-capacity of a BD-RW system, i. e. 25 GB, to allow one-to-onecopies. In recordable media mark formation occurs when a thresholdtemperature is exceeded. The mark-formation corresponds to anirreversible change in the information layer, and effects comparable tothe re-crystallization in rewritable media in principle do not occur.Thus, the marks that have been recorded in recordable media essentiallyreflect the optical/thermal profile that was present during therecording process. To reduce the mark length, thus to increase thetangential density, less laser power may be used to write a smallermark. As a consequence, the modulation of these smaller marks, and thussignal-to-noise ratio, will drop as well. This can be similarlydiscussed for write-once media based on phase-change material asrecording material. The as-deposited amorphous material willre-crystallize to form circularly shaped crystalline marks. Mark lengthreduction due to post heat is not possible.

U.S. Pat. No. 5,673,246 discloses an optical pick up device which makesa numerical aperture of an object lens in a radial direction of anoptical disc smaller than that in a tangential direction to a track onthe optical disc. Thus, a spot diameter in the tangential direction tothe track becomes smaller than that in the radial direction duringread-out. Thus, it is possible to prevent degradation MTF (ModulationTransfer Function) as well as to accurately read data recorded on thedisc.

It is therefore an object of the present invention to provide arecording apparatus and a corresponding recording method for recordingan information on a recordable optical record carrier by which datacapacities can be achieved that are similar to the data capacities ofcorresponding rewritable optical record carriers.

This object is achieved according to the present invention by arecording apparatus as claimed in claim 1, comprising:

-   -   a light source for generating a light beam,    -   optical means for irradiating said light beam onto said record        carrier,        wherein said optical means comprise means for reducing the        numerical aperture of said optical means in the direction        orthogonal to the information recording direction during        recording of information to obtain a light beam having a        substantial oval spot profile having a shorter axis in the        information recording direction compared to the direction        orthogonal thereto.

This object is further achieved according to the present invention by acorresponding recording method as claimed in claim 7.

The present invention is based on the idea to use a non-circularspot-profile for the recording process. If the spot-profile is elongatedin the radial direction, an oval-like thermal profile on the recordinglayer is obtained, so that marks can be written that are narrow intangential direction compared to their radial extent thus increasing theresolution. Such an oval spot profile is obtained according to thepresent invention by a reduction of the (effective) numerical aperturein the radial direction, i.e. in the direction orthogonal to theinformation recording direction on an optical record carrier, where theinformation is recorded along spiral tracks on the disc, is made duringthe recording process.

The proposed spot modification is also beneficial for high-speedwrite-once recording. At high recording speeds, thermal in-trackinterference may hamper the pit formation process. To reduce the thermalin-track interference, an ellipsoidal optical spot, such as obtainedwith the proposed modification, leads to less direct heating of thepreviously written pits.

Preferred embodiments of the invention are defined in the dependentclaims. An enhancement of the numerical aperture in tangentialdirection, i.e. in the information recording direction, during therecording process is beneficial as well, but more difficult to realizein practice since a lens re-design will be required.

Different preferred embodiments of the invention defining differentmethods and means for achieving the proposed reduction of the numericalaperture are claimed in claims 3 to 5. According to said embodiments,for instance, pupil filling, requiring no beam-shaper and properorientation of an-isotropic light-intensity of the laser, an oval pupilor a mask/diaphragm in the light path can be used to reduce thenumerical aperture.

In addition, as defined in claim 6, appropriate control means areprovided for switching said switchable means for reduction of thenumerical aperture or said switchable liquid crystal means on or off orfor bringing said means into the light path during recording and forremoving it thereafter, in particular during read-out or RW-recordingwhere spot-deformation is not desired.

An optical record carrier according to the present invention is definedin claim 8 which carries marks having a substantially oval profilehaving a shorter axis in the information recording direction compared tothe direction orthogonal thereto. Further, a computer program comprisingcomputer program means for crossing a computer to perform the steps ofthe method as claimed in claim 8 when said computer program is run on acomputer is defined in claim 9.

The present invention will now be explained in more detail withreference to the drawings in which

FIGS. 1 a, 1 b show long and short marks recorded on rewritable andrecordable record carriers with a circular spot profile,

FIGS. 2 a, 2 b show long and short marks recorded with a circular and anoval spot profile on recordable record carriers,

FIGS. 3 a, 3 b show an optical disc according to the present invention,

FIG. 4 shows a first embodiment of a recording apparatus according tothe present invention,

FIG. 5 shows different embodiments of diaphragms used according to thepresent invention,

FIG. 6 shows a second embodiment of a recording apparatus according tothe present invention,

FIG. 7 shows the optical spot profile used for read-out of data,

FIG. 8 shows the optical spot profile used for recording of data and

FIG. 9 shows the light intensity distribution of an oval spot in radialand tangential direction.

To achieve data capacities in write-once systems that are similar to thedata capacities of corresponding rewritable systems, very narrow (intangential direction t) marks need to be written at short runlengths,while in rewritable media the short marks are as broad (in radialdirection r) as the long marks. In recordable media the shortest marksare as broad as their lengths, i.e. have a circular shape. Thus, thewidth of the shortest marks decreases resulting in lower modulationduring read-out. This is illustrated in FIG. 1 showing a schematicdrawing of long (8T) and short (2T) marks recorded in rewritable (RW)media (FIG. 1 a) and recordable (R) media (FIG. 1 b) at high density. Itshall be noted that the shortest marks are comparable or smaller thanthe optical spot profile. The consequence is that during read-out, theoptical modulation of the shortest marks in recordable media vanishes.

This problem can also be illustrated as well by comparing the resolution(=2T_(pp)/8T_(pp)) of the 2T marks for BD-RW and BD-R at a density of 23GB. According to the BD-RW specification, the 2T resolution should belarger than 10%, and this is met in practice. However, so far even inthe best BD-R media the resolution at 23 GB is at most 5%. This poorresolution significantly deteriorates the jitter. The current inventionprovides a solution to write small but broad marks, i. e. having a hightangential density, such that a high modulation is preserved.

FIG. 2 shows a schematic drawing of long (8T) and short (2T) marks bothrecorded on a recordable medium. The marks shown in FIG. 2 a have beenrecorded using a circular spot shown on the right-hand side while themarks shown in FIG. 2 b have been recorded using an oval spot shown atthe right-hand side, said oval spot having a shorter axis in thetangential direction t compared to the axis in the radial direction r.As can be seen the marks shown in FIG. 2 b are narrow in the tangentialdirection t compared to their radial extent and compared to the marksshown in FIG. 2 a. Thus, the desired increase of the resolution can beobtained.

FIG. 3 shows a recordable optical disc according to the presentinvention in a top-view (FIG. 3 a) and in a cross-sectional view (FIG. 3b). The disc 2 comprises a number of guide grooves 13, which indicatethe direction in which the information is recorded on the disc 2, i.e.the information is recorded along said spiral grooves 13. Indicated arefurther in FIG. 3 the tangential direction t and the radial direction r.

FIG. 3 b shows as cross-sectional view the different layers of the disc2 which, in general, comprises a substrate layer 2 a, a recording layeror recording stack 2 b and a cover layer 2 c.

A first embodiment of a recording apparatus according to the presentinvention is schematically shown in FIG. 4. During recording or read-outthe disc 2 is rotated by rotating means 1. A light beam generated by alight source, in particular a laser diode 8, is focused onto the disc byan objective lens 3 after the light beam has passed a collimator lens 7,a polarizing beam splitter 6, a quarter-wave plate 5 and a diaphragm 4.During read-out the polarizing beam splitter 6 reflects light reflectedfrom the disc 2 through a lens 9 on a photo detector (array) 10 foranalyzing the reflected light. The general function of such a recordingapparatus is widely known and shall therefore not be explained furtherhere. Switching of the aperture's shape can be done mechanically orelectrically, in particular by electro-absorptive material or othermeans. This is indicated by corresponding control means 15.

FIG. 5 shows different embodiments of the diaphragm used in therecording apparatus shown in FIG. 4. FIG. 5 shows a circular diaphragm 4a used for reading data. FIG. 5 shows a diaphragm 4 b, essentiallyhaving a circular shape, but having two parallel straight sides, i.e. atwhich sides circular segments have been cut off. FIG. 5 shows anotherembodiment of a diaphragm 4 c having an oval shape. The diaphragms 4 band 4 c are used during recording of data. A reduction of the radialnumerical aperture is thus obtained by using a non-round aperturewherein the radial width x of the diaphragms 4 b, 4 c is a fraction ofthe tangential width y, in particular 0.7 y<x<y.

FIG. 6 shows another embodiment of a recording apparatus according tothe present invention. Therein a switchable beam-shaper 14 is providedin the light path between the collimator lens 7 and the PBS 6. In thisembodiment use is made of the intrinsic elliptic laser output incombination with the switchable beam-shaper 14 which can make the ovalbeam more or less round. In case of recording the beam-shaper 14 can be(partially) disabled, for instance electrically or mechanically, byappropriate control means 16 resulting in a lower radial rim-intensitywith respect to the tangential rim-intensity which is equivalent to areduced radial numerical aperture.

The laser diode 8 used to irradiate the disc 2 typically does not have auniform light intensity distribution, but a Gaussian distribution.Therefore, the light intensity at the rim of the objective lens 3 isusually lower than the light intensity in the middle of the objectivelens 3. The amount of relative rim-intensity, with respect to themiddle, is also indicated as “pupil filling”, i.e. indicates how muchlight fills the aperture of the lens 3. High rim intensity thus meanshigh filling. Furthermore, typical semiconductor lasers have differentwidths of the Gaussian distribution in orthogonal directions. To obtainnearly equal rim intensity at a round aperture a so-called beam-shaperis used.

The focussed optical spot profile and marks/pits obtained by therecording method according to the present invention in a groove G (13)separated from another groove G by a land area L are shown in FIG. 7.During reading along the information recording direction t (=tangentialdirection) a circular optical spot profile 11 a is used. Shown are alsothe recorded marks 12 having an oval profile.

FIG. 8 shows the optical spot profile 11 b used for recording of data.As can be seen an oval spot profile 11 b is used resulting in oval marks12. The light intensity of such an oval spot 11 b used for recording andgenerated by the semiconductor laser 8 is shown in FIG. 9. As can beseen both in radial and tangential direction the light intensity shows aGaussian distribution.

The present invention provides a solution to obtain higher datacapacities when recording in particular on recordable optical recordcarriers. The numerical aperture of the optical means for directing alight beam on the optical record carrier during recording is reducedaccording to the present invention to obtain a light beam having asubstantially oval spot profile during recording, in particular having asmaller width in the tangential direction compared to the width inradial direction.

1. A recording apparatus for recording an information on a recordableoptical record carrier by irradiation of a light beam onto said recordcarrier for forming marks and lands representing said information alongan information recording direction, comprising: a light source forgenerating a light beam, optical means for irradiating said light beamonto said record carrier, wherein said optical means comprise means forreducing the numerical aperture of said optical means in the directionorthogonal to the information recording direction during recording ofinformation to obtain a light beam having a substantial oval spotprofile having a shorter axis in the information recording directioncompared to the direction orthogonal thereto.
 2. A recording apparatusas claimed in claim 1, wherein said optical means are adapted forincreasing the numerical aperture of said optical means in theinformation recording direction during recording of information.
 3. Arecording apparatus as claimed in claim 1, wherein said means forreducing the numerical aperture comprise a switchable non-round, inparticular oval, aperture in the light path from the light source tosaid record carrier during recording.
 4. A recording apparatus asclaimed in claim 3, wherein the short axis of said aperture is by afactor of 0.7 to 0.99 shorter than the long axis.
 5. A recordingapparatus as claimed in claim 1, wherein said means for reducing thenumerical aperture comprise a switchable beam-shaper in the light pathfrom the light source to said record carrier during recording to obtaina reduced rim-intensity of the light beam in the direction orthogonal tothe information recording direction.
 6. A recording apparatus as claimedin anyone of claims 3 to 5, further comprising a control means forcontrol of said switchable means by switching said switchable means onor off by bringing said switchable means into the light path duringrecording.
 7. A method of recording an information on a recordableoptical record carrier by irradiation of a light beam through opticalmeans onto said record carrier for forming marks and lands representingsaid information along an information recording direction, wherein thenumerical aperture of said optical means is reduced in the directionorthogonal to the information recording direction during recording ofinformation to obtain a light beam having a substantially oval spotprofile having a shorter axis in the information recording directioncompared to the direction orthogonal thereto.
 8. An optical recordcarrier carrying an information recorded by irradiation of a light beamonto said record carrier through optical means for forming marks andlands representing said information, said marks having a substantiallyoval profile having a shorter axis in the information recordingdirection compared to the direction orthogonal thereto.
 9. Computerprogram comprising computer program means for causing a computer toperform the steps of the method as claimed in claim 8 when said computerprogram is run on a computer.